In general, it is required to optimize rigidity balance of a center part and a shoulder part of a tire of a motorcycle in order to achieve stability both in running straight and in turning because a grounding part shifts largely from the center part to the shoulder part in cases of running straight to turning.
Conventionally, although a method of enhancing a grounding performance by adopting a so-called mono-spiral belt (MSB) structure for a rear tire has been taken as means for improving the stability of the motorcycle, the mono-spiral belt structure has been apt to be pointed out that it lacks stability due to a lack of lateral rigidity for motorcycles having a heavy weight and large horse power. Therefore, a method has been taken so as to improve convergence of vibrations after input of disturbance by changing a cord material of the belt from Kepler to steel whose rigidity is higher and which enhances lateral rigidity of a tread.
However, it has been unable to assure enough stability by the above-mentioned methods because flexural rigidity of the tread part is also enhanced and a lack of lateral force occurs due to a lack of grounding area in turning when an especially large lateral force is required.
As technologies of improving stability of a tire of a motorcycle by enhancing the rigidity, Patent Document 1 describes a pneumatic radial tire for a motorcycle whose controllability and stability are remarkably improved without lowering driving comfortableness by assuring homogeneous and adequate tire-case rigidity across the whole range of a tire side portion by adopting a structure of winding up a first carcass ply to an end of a belt and sandwiching and arresting it between a belt end region and the carcass layer and by extending a second carcass ply at least from the belt end region to a bead portion beyond position of a maximum width of the carcass ply. Further, for the purpose of achieving both comfortableness in running straight and a feeling of rigidity in turning, Patent Document 2 describes a tire for a motorcycle having belt layers disposed on the outside of a carcass in a tire radial direction and within a tread part, wherein the carcass has a continuous toroidal ply and a split ply, that is hollow in a tread center part, having an outer edge in a tire radial direction on the outside of a tire equator in a tire axial direction and on the inside of an outer edge of the belt layer in the tire axial direction and composed of a pair of right and left split ply pieces extending from the outer edge through the inside in the tire radial direction to a bead core.
Patent Documents 3 and 4 also describe technologies for improving stability of a radial tire for a motorcycle having a carcass and a belt layer in which cords are wound in spiral at low angle with respect to a tire equator.
Patent Document 1: Japanese Patent Laid-open No. 2000-211317 gazette (Claims and others)
Patent Document 2: Japanese Patent Laid-open No. 2003-39916 gazette (Claims and others)
Patent Document 3: Japanese Patent Laid-open No. 5-178004 gazette (Claims and others)
Patent Document 4: Japanese Patent Laid-open No. 5-246210 gazette (Claims and others)